Electromagnetic wave coupling devices



' 'May 9, 1967 E. H. SHIVELY ETAL 3,319,190

ELECTROMAGNETIC WAVE COUPLING DEVICES Filed July 2,- 1962 FOR THE ODD MODE (FIG.5)

K m a 7% n d J m U E a y W United States Patent Office 3 ,3 l h, i 90 Patented May 9, 1967 3,319,190 ELECTROMAGNETIC WAVE COUPLING.

DEVICES Edward H. Shively, Raymond, Maine, and Clarence W.

Jones, Hudson, N.H.; said Shively assignor to Dielectric Products Engineering Company, Inc., Raymond, Maine, a corporation of Michigan, and said Jones assiguor to Massachusetts Institute of Technology, Cambridge, Mass., a corporation of Massachusetts Filed July 2, 1962, Ser. No. 212,500

6 Claims. (Cl. 33310) (Filed under Rule 47(a) and 35 U.S.C. 116) The invention relates to circuits operative at microwave frequencies and more particularly to a novel and improved coupling device.

In microwave circuitry apparatus for coupling electromagnetic energy between two circuits finds frequent use. A wide variety of coupling devices are known. However, in the relatively low microwave frequencies (in the order of 15 megacycles and above), the nature of coupling devices capable of handling substantial amounts of power is severely limited, and such devices become quite massive in size. In some applications, for example in protective device employed with a higher power airborne radar system, the system operating at the frequency range of 90-160 megacycles, the space necessary for a coupler of a conventional design of this type, is not infrequently unavailable. I Accordingly, it is an object of this invention to provide a novel and improved electromagnetic energy coupling device of compact configuration that employs distributed constant transmission line circuitry operable over a relatively broad band of frequencies.

In accordance with the invention there is provided an electromagnetic energy coupling device having a pair of transmission lines of helical configuration that are dis posed relative to a cylindrical ground plane. In this device two transmission systems are coupled together in a continuous fashion in which the normal modes of the transmission sections (two TEM (transverse electromagnetic) modes) are unmatched at the transitions. In the apparatus of the invention these modes are propagating along helical paths in which the effective path lengths of the two modes are different, and in the embodiment disclosed in greater detail hereinafter the odd mode has a shorter path length than the even mode. Dielectric loading distributed along the entire path length is employed to decrease the velocity of propagation of the odd mode so that the effective electrical length of that mode is increased to make it equal to the path length of the even mode. In this manner the two modes are maintained in tracking relation to provide the required coupling and d-irectivity characteristics. The coupled conductors and the effective TEM mode path lengths are approximately ,equal to an odd number of quarter wavelengths at the design center frequency.

The invention provides a compact coupling device which may be of the hybrid type (three db coupler) or other coupling relation in a unit that is about one fifth the length of the conventional coupler of this type. The device has flexibility in application as there is freedom of choice in the radial location of the input and output configuration. Couplers constructed in accordance with the invention up to frequency ranges at which the circumference of the ground plane approaches a wavelength in dimension. In a one kilowatt three db coupler structure constructed in accordance with the invention and operative in the -460 megacycle frequency range, the outer dimensions of the cylindrical casing is approximately three inches in diameter and six inches in length, in which are positioned two Ms" x /8" conductors in helical configuration.

Other objects, features and advantages of the invention will be seen as the following description of a preferred embodiment thereof progresses, in conjunction with the drawings, in which:

FIG. 1 is a perspective view of the completed coupler structure;

FIG. 2 is a diagrammatic view of the nature of the coupling relation of the device;

FIG. 3 is a sectional view of a portion of the two coupled transmission lines showing their relation to the sup port structure and the ground plane;

FIG. 4 is a perspective view of the coupler structure with the ground plane removed;

FIGS. 5 and 6 are diagrammatic views illustrating the configuration of the two TEM propagation modes; and

FIG, 7 is a graph showing the coupling relation of the structure constructed in accordance with principles of the invention.

The coupler as shown in FIG. 1 is a four port device having '%a" coaxial terminals 10, 12, 14, 16 connected into a cylindrical chamber 18. The chamber 18 functions as a ground plane. The terminals 10 and 14 are connected to a first transmission line section 20 in the chamber and terminals 12 and 16 are connected to a second transmission line section 22 having the general coupling relation as indicated in FIG. 2. Rather than being disposed in straight lines as shown in FIG. 2, however, the transmission lines 20, 22 are of helical configuration and are disposed on a dielectric support member 24 (FIGS. 3 and 4). The spacing of the helical turns in this coupler is about two inches with one eighth inch between the coupled lines. Closer turn spacing may be employed down to a distance in the order of the width of the helical conductors and even closer if bafiiing is employed to prevent cross coupling between adjacent turns.

There are two TEM modes of energy propagation in this coupling device, an odd mode indicated in FIG. 5 in which the potentials on the two conductors 20, 22 at the same point along their length are of opposite polarity and an even mode (diagrammatically indicated in FIG. 6) in which the polarity of the potential on both conductors 20, 22 at the same point along their length is the same. These modes of propagation produce magnetic fields as indicated generallyin FIGS. 5 and 6. The magnetic field in the odd mode of propagation is generally symmetrical about the conductors 20, 22 (slight distortion is due to the proximity of the ground plane 18) so that its effective path is through point A approximately at the center of the width of the conductors. However, in the even mode of propagation a different magnetic field configuration is established and the effective path is nearer the ground plane, passing approximately through point B between the outer edges of the conductors 20, 22 and the ground plane 18.

In order to provide the proper coupling of signals the effective length of these two modes of propagation must be equal. The path length of the odd mode, however, is shorter than the path length of the even mode due to the greater radius of the even mode. By loading one of the propagating modes, in this case by the insertion of a suitable microwave loading material 26 such as polyethylene or polypropylene in the configuration as indicated in FIG. 3, the effective electrical path length of the odd mode is increased to a value equal to that of the even mode. This dielectric material 26 is inserted between the two conductors 20, 22 at their points of smallest radius. Dielectric loading in this manner slows down the velocity of propagation of the odd mode so that the effective electrical path lengths of the two TEM modes are equal. The relation between dielectric load ng and the velocity of propagation as set forth in the following equation:

Where V is the velocity of propagation, ,u. is the permeability and e is the dielectric constant. By loading the odd mode in this manner the electromagnetic field is distorted and the velocity of propagation of the odd mode along the conductors is reduced.

In this case of uniform coupled lines in this type of coupler, the output voltage ratio V /V =K sin 5L, where K equals one-half (k-1/ k) and k equals 2Z /R =2R /Z [i is the transmission constant (Zn/h); and L is the length over which the lines are coupled. The resulting power transfer characteristics are indicated in FIG. 7 for a coupling length of )\0/ 4 (a quarter wavelength of the center frequency) with the coupling between these two lines adjusted to be slightly tighter than 3 db at the center frequency (f so that a point of exactly 3 db coupling is at each side of the center frequency and a useful band width of about two to one (depending on the deviation from 3 db which can be tolerated) is available. Utilizing this principle lines longer than the quarter wavelength may be also coupled, for example, by employing three sections, and making the coupling between the two lines in the center sec-- tion different from the coupling in the end sections.

By employing the helical configuration of coupled conductors the overall length of the coupler unit can be reduced by the factor of five times (two conductors of thirty inches coupling length are mounted in a ground plane cylinder chamber having an overall length of six inches). In a typical device the diameter of the ground plane cylinder is three inches and conductors /8" x in cross section couple ten kilowatts of peak power and one kilowatt of average power.

While a preferred embodiment of the invention has been shown and described various modifications will be obvious to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

We claim:

1. A distributed parameter coupler comprising two electrical transmission lines of helical configuration, each line being positioned with its helical axis coincident with the helical axis of the other line so that the two lines are in electromagnetically close coupled relation over a continuous length, the spacing between adjacent helical turns of each said line being such that there is no substantial electromagnetic coupling between said adjacent turns,

a cylindrical ground plane adjacent said two transmission lines, I

said coupler being arranged for operation at a frequency to provide odd and even transverse electromagnetic modes of propagation along said two transmission lines,

and means for loading one of said modes of propagation to make the electrical path lengths'of the two modes of propagation along said conductors equal. 2. The coupler as claimed in claim 1 wherein said means for loading one of said modes is a fluorocarbon polymer (Teflon).

3. Electromagnetic coupling apparatus comprising two electrical transmission lines,

each line being formed in a helical configuration and positioned with its helical axis coincident with the helical axis of the other line so that the two lines are in electromagnetically coupled relation over a continuous length, the spacing between adjacent helical turns of said lines being such that there is no substantial electromagnetic coupling therebetween, said coupler being arranged for operation at a frequency to provide odd and even transverse electromagnetic modes of propagation along said two transmission lines,

and propagation mode loading means disposed relative to said lines to make the electrical path lengths of modes of propagation along said transmission lines equal.

4. A distributed parameter coupler comprising two electrical transmission lines,

each said line being formed in helical configuration with their helical axes coincident so that the two lines are in electromagnetically close coupled relation over a continuous length, the spacing between adjacent helical turns of each said line being such that there is no substantial electromagnetic coupling between said adjacent turns,

a cylindrical ground plane surrounding said two transmission lines,

said coupler being arranged for operation at a frequency to provide odd and even transverse electromagnetic modes of propagation along said two transmission lines,

and dielectric means for loading one of said modes of propagation to make the electrical path lengths of the two modes of propagation along said conductors equal. 5. A distributed para-meter coupler comprising two electrical transmission lines of identical cross sectional configuration,

each said line being formed in a helical configuration of uniform inner and outer diameter,

cylindrical support means for mounting said lines in continuous electrically coupled relation with their helical axes coincident so that the two lines are in electromagnetically close coupled relation over a continuous length, the spacing between adjacent helical turns of each said line being such that there is no substantial electromagnetic coupling between said adjacent turns,

a cylindrical ground plane adjacent said two transmission lines,

said couplire being arranged for operation at a frequency to provide odd and even transverse electromagnetic modes of propagation along said two transmission lines with respect to said cylindrical ground plane,

and means for loading one of said modes of propagation to make the electrical path lengths of the two modes of propagation along said conductors equal.

6. A distributed parameter coupler comprising two electrical transmission lines of identical cross-sectional configuration,

each said line being formed in a helical configuration of uniform inner and outer diameter, cylindrical support means for mounting said lines in continuous electrically coupled relation with their helical axes coincident,

a cylindrical ground plane adjacent said two transmis- .sionlines,

said coupler being arranged for operation at a frequency to provide odd and even transverse electromagnetic modes of propagation along said two transmission lines with respect to said cylindrical ground plane, the electrical path length of said even mode being longer than said odd mode,

and a dielectric member interposed between said two coupled lines along their length for decreasing the velocity of propagation of the odd mode to make the propagation along said conductors equal.

References Cited by the Examiner UNITED STATES PATENTS Larson 333-90 White 333--31 White 333-31 Lokatos 333-31 Holland 333-31 HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner. 

1. A DISTRIBUTED PARAMETER COUPLER COMPRISING TWO ELECTRICAL TRANSMISSION LINES OF HELICAL CONFIGURATION, EACH LINE BEING POSITIONED WITH ITS HELICAL AXIS COINCIDENT WITH THE HELICAL AXIS OF THE OTHER LINE SO THAT THE TWO LINES ARE IN ELECTROMAGNETICALLY CLOSE COUPLED RELATION OVER A CONTINUOUS LENGTH, THE SPACING BETWEEN ADJACENT HELICAL TURNS OF EACH SAID LINE BEING SUCH THAT THERE IS NO SUBSTANTIAL ELECTROMAGNETIC COUPLING BETWEEN SAID ADJACENT TURNS, A CYLINDRICAL GROUND PLANE ADJACENT SAID TWO TRANSMISSION LINES, 